Neurotrophins, and more particularly Nerve Growth Factor (NGF), are critical for the differentiation and survival of specific neuronal populations during development, and modulate neural plasticity in the mature nervous system [1, 2]. Paradoxically, NGF has also been described as inducing apoptosis of neurons during development and eliminates damaged neurons and glial cells in pathological conditions [3, 4]. NGF has been described as a mediator of tissue inflammation and chronic pain [5], and as accumulating in several pathologies undergoing neuroinflammation [6-8].
NGF exerts its actions through two non-related transmembrane receptors, the tyrosine kinase receptor TrkA, and the p75 neurotrophin receptor (p75NTR). TrkA is a tyrosine kinase receptor that activates well-characterized signalling pathways promoting neuronal survival, differentiation and plasticity [2, 9, 10], whereas p75NTR is a member of the tumor necrosis factor receptor superfamily that can act as a death receptor signalling apoptosis [4, 11]. In addition, p75NTR can also act as a co-receptor for Trk A, B, and C, or interact with other receptors (sortilin, Nogo-R) to modulate diverse biological effects, including survival, cytoskeleton rearrangement and axonal elongation [4, 12].
Hence, native neurotrophins, such as NGF, have been described as capable of inducing p75NTR-dependent apoptosis.
p75NTR is highly expressed in motor neurons at the embryonic stage, but its expression levels gradually ends after birth [13]. Neither TrkA nor p75NTR are expressed by adult motor neurons, although p75NTR can be re-expressed following axotomy [14-16] and in pathological conditions involving motor neuron degeneration, such as amyotrophic lateral sclerosis (ALS) [17, 18]. Furthermore, p75NTR has been implicated in motor neuron death induced by axotomy [14, 19, 20]. Abnormal expression of p75NTR and NGF may contribute to adult motor neuron death observed in ALS transgenic mice overexpressing mutant Cu—Zn superoxide dismutase (SOD-1) [18, 21-25].
More recently, pro-neurotrophins have emerged as potent inducers of p75NTR-dependent apoptosis by signalling through a complex formed by p75NTR and sortilin [14, 15].
For example, WO 2005/014039 expresses and follows this “pro-neurotrophin” hypothesis, without envisioning the possibility of any post-translational modification of the mature neurotrophin: see e.g., page 46 lines 24-28 of the PCT application as published, which states <<because endogenous mature NGF were found in only extremely low concentrations in the tissue extracts and nearly failed to meet the detection limit in culture media, NGF precursors (19-21, 28 and 32 kDa) are the more likely mediators of apoptosis for p75NTR-expressing motor neurons>>.
Hence, before the invention, the main hypothesis was that pro-neurotrophin, such as pro-NGF, was the endogenous factor leading to neuron apoptosis.